This invention relates to the polymerization of tetrafluoroethylene monomer with fluoroalkyl perfluorovinyl ether monomer in a perfluorinated or saturated nonperfluorinated fluorocarbon solvent in the presence of a chain transfer agent.
Prior to this invention copolymers of tetrafluoroethylene and fluoroalkyl perfluorovinyl ether have been polymerized in various non-aqueous media. The polymers formed in these non-aqueous systems contain acid fluoride end groups that result from the rearrangement of the fluorovinyl ether radical on the end of the growing polymer chain. The rearrangement reaction takes place by the mechanism. ##STR1##
This rearrangement results in the terminal of the polymerization of that chain by the formation of an acid fluoride end group and a new free radical group. During storage, these end groups are hydrolized and decompose during extrusion forming gases which show up as bubbles in extruded products. This is obviously undesirable. These end groups can be stabilized by use of the humid heat treatment process of U.S. Pat. No. 3,085,083 entitled "Stabilized Tetrafluoroethylene-Fluoroolefin Copolymers Having CF.sub.2 H End-Groups" which converts the unstable acid end groups into stable-CF.sub.2 H end groups. The major disadvantages of the humid heat treatment are that it is slow, adds considerable cost to polymers that are treated in this manner and also tends to add or allow contamination of the polymer with dust and other particles which may be introduced in the heat-treatment process.
Another problem with tetrafluoroethylene/fluorovinyl ether (TFE/FVE) copolymers is their tendency to swell on being extruded through small orifices into tubes, wire coating, and the like. This swelling causes problems in dimension control of the finished parts but, worse than that, it causes excessive shrinkage of the parts when they are reheated near the melting point. High swelling resins have a highly shear stress dependent viscosity indicating a broad molecular weight distribution. The higher the swelling, the broader the molecular weight distribution at the same melt viscosity. The reason for this swelling is that the polymer is viscoelastic and some of the energy put in to cause flow results in elastic or recoverable deformation. It is this elastic recovery which causes the swelling of the polymer as it emerges in viscous flow from an orifice. A polymer with a broad molecular weight distribution contains at equal melt viscosity, a larger proportion of very high molecular weight molecules, which have large elastic components, than a polymer with narrow molecular weight distribution. Thus, the former polymer would be expected to swell to a greater degree than the latter polymer. In certain applications of tetrafluoroethylene-fluorovinyl ether copolymers it is highly desirable that the resin undergo little shrinkage when heated near its melting point. A specific embodiment of this invention on tetrafluoroethylene/fluorovinyl ether copolymers, prepared in the presence of methanol as chain transfer agents, is their considerably reduced tendency to swell upon being extruded and consequently to shrink when heated near their melting point. As was stated above, it is believed that the reduction in the swelling tendency of the copolymer prepared in the presence of methanol is due to its narrower molecular weight distribution. Another advantage of the polymers of this invention is their improved toughness as indicated by their MIT flex life. The MIT flex life normally increases with melt viscosity and fluorovinyl ether content of the polymer. Thus, if the fluorovinyl ether content is held constant, the MIT flex life can be increased by increasing the melt viscosity of the polymer. Similarly, if the melt viscosity is held constant, the MIT flex life can be increased by increasing the fluorovinyl ether content of the polymer. We have found that the MIT flex life is increased for polymers with the same melt viscosity and fluorovinyl ether content when they are prepared in the presence of methanol. It is believed that the increase in toughness of the polymers prepared in methanol is also due to their narrower molecular weight distribution relative to polymers prepared in the absence of methanol. Melt viscosity is a function of both weight average and number average molecular weights while toughness is primarily a function of number average molecular weight. If the molecular weight distribution is narrowed, the ratio between weight average and number average molecular weight will be less. Thus, at the same melt viscosity, the polymers with a narrower distribution will have a higher number average molecular weight and consequently higher toughness. In certain applications of tetrafluoroethylene/fluorovinyl ether copolymers it is highly desirable that the resins have high toughness but still have low enough melt viscosity (1-100.times.10.sup.-4 poises) for easy fabrication and contain the minimum amount of the expensive fluorovinyl ether to be commercially attractive. This is particularly important in applications such as tank linings and thin walled tubing which require high stress crack resistance.
As discussed in U.S. Pat. No. 3,085,083, to Schreyer, carboxylate end-groups in the fluorocarbon polymer chain are the principle cause of the instability of fluorocarbon polymer at melt fabrication temperatures. Since acid fluoride end groups. ##STR2## are the result of the rearrangement of the fluorovinyl ether on the end of the growing chain and since these are easily converted to carboxylic acid end groups it can easily be seen that this type of chain termination will result in polymer instability.
Since it is known that the number of unstable end groups formed on TFE/FVE copolymers decreases as the molecular weight increases, one would expect that decreasing the amount of initiator to produce high molecular weight polymer would decrease instability and increase the MIT flex life of the polymer. This of course happens, but there is also a large decrease in the ease of fabricability of the polymer. Addition of a hydrogen containing chain transfer agent to the polymerization recipe reduces the number of chain terminations that are made by rearrangements of the FVE monomer and increases the number of chain terminations such as those by the mechanism EQU CF.sub.2 --CF.sub.2.+HCH.sub.2 OH.fwdarw. CF.sub.2 CF.sub.2 H+.CH.sub.2 OH
The end groups formed by the chain transfer agent are stable hydride end groups (--CF.sub.2 H), the same end groups that result from the patented Schreyer process. The resulting polymer has approximately the same number of unstable acid fluoride end groups as a much higher molecular weight polymer made by a process without the chain transfer agent present.